1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, used in image forming processes such as electrophotography, electrostatic recording and electrostatic printing. It also relates to a one-component type developer having the toner, and a two-component type developer having the toner and a carrier.
2. Related Background Art
A large number of methods have been hitherto known as electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (U.S. Pat. No. 3,666,363) and Japanese Patent Publication No. 43-24748 (U.S. Pat. No. 4,071,361) and so forth. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and according to various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat or pressure.
Developing methods by which the electrostatic latent image is formed into a visible image are also known in variety. A large number of such developing methods are known, as exemplified by the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the fur brush development and the liquid development.
Of these developing methods, the magnetic brush development, the cascade development and the liquid development, making use of a two-component developer mainly composed of a toner and a carrier, are widely put into practical use. There is also a method in which a developer container holding a toner and magnetic particles used for applying the toner, a toner carrying member from which the toner is transported to a latent image bearing member and a magnet that forms a magnetic brush by the aid of the magnetic particles used for applying the toner, coming into contact with the toner carrying member at an upstream side of a toner outlet of the developer container are provided so that a thin layer of the toner is formed on the toner carrying member, where a gap between the toner carrying member and the latent image bearing member are set larger than the thickness of the toner layer and the electrostatic latent image is developed at a developing zone defined by the gap.
These methods require keeping the toner in a state such that it is uniformly mixed with the carried at the developing zone or in a state where it is uniformly thinly applied to the the toner carrying member. As a force to achieve such a state, it is predominantly an electrostatic attraction force and a physical adhesion force. That is, it becomes necessary to precisely control electrostatic charges and triboelectric chargeability possessed by the toner.
However, it is difficult to uniformly control the electrostatic charges possessed by the toner and also make the electrostatic charges stable after repeated copying over a long period time or in a special environment of high temperature and high humidity or low temperature and low humidity.
Various proposals are made for a developing method making use of a one-component developer comprised of only the toner, which can avoid the problems involved in the developing method making use of the two-component developer. In particular, there are many advantages in methods employing a developer comprising toner particles having magnetic properties.
As toners used in these developing methods, fine powders comprising a dye or pigment dispersed in a natural or synthetic resin were previously used. It is also known to use a developing fine powder to which a third material has been added for various purposes.
The toner image having been developed is fixed to a transfer medium such as paper if necessary. In relation to this step of fixing the toner image to a transfer medium, various methods or techniques have been advanced. A method most commonly available at present is the pressure heating system making use of a heating roller. The pressure heating system making use of a heating roller is a method of carrying out fixing by causing a transfer medium to pass over a heating roller whose surface is formed of a material with a releasability to toner while a toner image surface of the former is brought into contact with the surface of the latter under application of a pressure. Since in this method the surface of the heating roller comes into contact with the toner image of the transfer medium under application of a pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the transfer medium, so that fixing can be carried out rapidly. In this method, however, since the surface of the heating roller comes into contact with the toner image under application of a pressure in the latter's molten state, part of the toner image may adhere and transfer to the surface of the fixing roller, which may re-transfer to the subsequent transfer medium to cause an offset phenomenon, resulting in a contamination of the transfer medium. Thus, it is sought to make an advancement in resins for toners having much higher low-temperature fixing performance and high-temperature anti-offset properties.
Moreover, since copying machines hereafter are to be made higher in speed, toners must now provide more highly enhanced fixing performance to paper, a high resolution and high-speed development and a high running performance.
Under such circumstances, a developer comprised of a toner using a cross-linked polyester as a binder resin and a resin-coated carrier is proposed in Japanese Patent Applications Laid-open No. 62-127748 and No. 62-127749. This developer, however, has a problem that, in its evaluation made on image reproduction in an environment of low humidity, the charge performance of the toner becomes unstable when the developer is mechanically agitated in a developer assembly.
Japanese Patent Application Laid-open No. 58-11953 discloses a toner for developing electrostatic latent images that is characterized by a binder resin comprising a polyester resin containing not less than 5% by weight of chloroform-insoluble component and containing an anti-offset agent comprised of a non-polar substance and a polar substance.
Japanese Patent Application Laid-open No. 62-78569 discloses a toner making use of a polyester having on its side chain a saturated or unsaturated aliphatic hydrocarbon group having 3 to 22 carbon atoms.
In these toners, however, because of a poor compatibility of the polyester resin with a polyolefin wax, poor dispersion of the polyolefin wax tends to occur when the toners are prepared, often resulting in generation of free polyolefins during pulverization. Taking into account the future progress toward high-speed copying machines, this raises the problem that occurrence of faulty cleaning or deterioration of anti-offset properties may result. With regard to fixing performance in an environment of low temperature and developing performance in an environment of low humidity, it is hard to say that they are satisfactory in high-speed copying machines.
Japanese Patent Applications Laid-open No. 2-129653 and No. 3-46668 disclose toners using as a binder resin a polyester obtained by treating a polyester resin with an acid or an alcohol.
These toners are certainly effective for improving fixing performance or making triboelectric charges stable. However, because of the use of a monoalcohol having a carbon atom number as small as 10, poor dispersion of polyolefin wax tends to occur, so that, taking into account the future progress toward high-speed copying machines, the occurrence of faulty cleaning or deterioration of anti-offset properties may come into question. Also with regard to the fixing performance in an environment of low temperature and the developing performance in an environment of low humidity, it is hard to say that they are satisfactory.
In the prior art discussed above, the fixing and the anti-offset properties can be balanced with difficulty because of the poor compatibility of a polyester with a polyolefin wax, so that, when a developer carrying member has a high process speed as in the case of high-speed copying machines, the charges on the toner can be controlled only with difficulty. Thus, in future progress toward high-speed copying machines, none of them have still reached a satisfactory level for improved fixing performance, anti-offset properties and developing performance.
In addition, in recent years, it is sought to make copied images have higher image quality by making copying machines digital and making toner particles finer. However, even if the resolution or sharpness of images can be increased by making toner particles finer, various problems may occur. In the first place, making toner particles finer results in a poor fixing performance at halftone areas. The reason therefor is that it is hard for a fixing pressure to be applied to the toner transferred to concavities on the surface of a transfer medium and the amount of heat imparted thereto is smaller, and also that the toner transferred to convexities on the surface of the transfer medium has a smaller toner layer thickness and hence a shear force applied thereto per each toner particle is larger which tends to cause an offset phenomenon.
Moreover, such a toner which has fine particles with a small particle diameter, when used in high-speed copying machines, may acquire excessive charges especially in an environment of low humidity and often causes fog or a decrease in density.
In multi-functional copying machines having functions such that multiple multicolor copies are taken by, e.g., previously erasing part of an image by exposure or the like and then inserting another image to that part or where the margin of a copy sheet is framed out, the part to be left white on the image may be fogged.
In other words, there may occur a problem that when a potential with a polarity reverse to that of a latent image is applied to the development standard potential by the use of strong light such as light of an LED or a fuse lamp to erase the image, the part thus erased strongly tends to be fogged.
In addition, even if the resolution or sharpness of images can be improved by making copying machines digital or making toner particles smaller in diameter, various problems may arise.
In the first place, there is the problem of fogging as stated above. As toner particles are made smaller in diameter, the surface areas of the toner increases, therefore resulting in a large breadth of charge distribution which tends to cause fogging. The increase in the surface area of the toner may also make charging performance of the toner more likely to be affected by environment. When toner particles have a smaller diameter, it is clear that the state of dispersion of a magnetic material or colorant or a release agent has a great influence on the charging performance of the toner.
As for digital copying machines recently available, in a photographic image with letters or characters, it is required for the letters or characters of its copied image to be sharp and for the photographic image to be reproduced in a density gradation faithful to the original. In general, in the copying of an image with letters or characters, not only the density gradation of the photographic image may be impaired as the density of letter or character lines is made higher in order to obtain sharp letters or characters, but also a very coarse image may result at its halftone portion. Moreover, since the toner is applied in a larger quantity as the density of letter or character lines is made higher, what are called blank areas caused by poor transfer may instead occur, which is a phenomenon in which the toner on a line or lines has come off, resulting in a copied image with a low image quality. On the other hand, an attempt to improve the density gradation of the photographic image may cause a decrease in the density of letter or character lines, resulting in poor sharpness.
In recent years, the density gradation has been improved to a certain extent by reading image density and converting it into digital signals. Under the existing conditions, however, its improvement can not be said to be satisfactory.